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Targeted CD123 therapy in ALL
B-ALL cell lines that expressed CD123 at levels similar to those of blasts from B-ALL patients (ABC values: 457- 3,741) and encompassed genetic variants associated with a poor prognosis (Table 2). IMGN632 was highly cytotoxic to B-ALL cell lines with half maximal inhibitory concentra- tions (IC50) between 0.6 and 20 pM. The conjugate cytotox- icity was CD123-specific, as dosing with a non-targeting conjugate (Ab-DGN549) prepared with a similar linker and payload reduced the potency more than 1000-fold.
IMGN632 potency was evaluated in primary samples from eight patients with B-ALL. The blast cells in each of the untreated samples proliferated in culture, as manifested by a dramatic decrease in the cell trace violet fluorescent signal, while only a small shift in fluorescence (likely due to nonspecific dissociation of the dye) was observed in the lymphocyte population, suggesting lack of proliferation (Figure 2A). IMGN632 eliminated more than 90% of the B- ALL blast population in five of eight samples, including those from newly diagnosed as well as relapsed/refractory patients, at low pico-molar concentrations (Figures 2B,C). Normal lymphocytes were not affected by IMGN632 at 100-fold higher concentrations.
Discussion
In this study, we demonstrate pervasive CD123 expression in a large cohort of ALL patients. In B-ALL, previously reported to be often CD123+,13,15,18 our findings demonstrate that CD123 expression translates into sus- ceptibility to targeting CD123 via the antibody-drug con- jugate IMGN632 in vitro and ex vivo. Coupled with previ- ously published data on the differential overexpression of CD123 on the surface of acute myeloid leukemia blasts, but not on normal hematopoietic stem cells,25 these findings provide a plausible basis for the explo- ration of anti-CD123 therapy in B-ALL patients in whom frontline treatment fails. Furthermore, in T-ALL our find- ings suggest that CD123 expression might be associated with an early precursor immunophenotype, including ETP-ALL. Since the latter is a high-risk subset, targeting CD123 in these patients might be of potential benefit if the association is confirmed in a larger group of ETP-ALL patients.
Previously, the CD123-targeting conjugate IMGN632 was reported to have high potency in pre-clinical models of acute myeloid leukemia.24 In this study we demon- strated the efficacy of IMGN632 in pre-clinical B-ALL models, suggesting that IMGN632 is a promising anti-
leukemia agent in CD123-expressing ALL. In addition to its promising anti-leukemia activity, CD123 offers an alternative to CD19-targeting therapeutic approaches to circumvent CD19 antigen-loss relapses and/or elimina- tion of CD19– stem cells.26
B-ALL is a heterogeneous disease that includes nine genetic subgroups recognized in the World Health Organization classification.27 Data on correlations between CD123 expression and B-ALL subgroups are limited. Djokic et al. showed that B-ALL cases with a hyperdiploid karyotype have significantly higher rates of CD123 expression compared to other subgroups such as BCR/ABL1, MLL, ETV6/RUNX1 and diploid karyotype.18 However, their study included mostly pediatric patients, and the BCR/ABL1 subgroup formed a small proportion of their cohort (2/81 pediatric patients; 3/13 adult patients). In this study, we identified a significant corre- lation between CD123 expression and the presence of the BCR/ABL1 fusion, with a higher percentage of CD123+ events and a higher intensity in comparison to the Ph– B-ALL and T-ALL groups. The association between the BCR/ABL1 fusion and CD123 has also been identified in chronic myeloid leukemia, in which target- ed CD123 inhibition appears to deplete chronic myeloid leukemia progenitor and stem cells.28,29 The findings in this study suggest a close correlation between BCR/ABL1 and CD123, this time in the context of B-ALL. Whether the correlation between Ph– B-ALL and CD123 is related to an underlying Ph-like biology remains to be deter- mined and is being pursued in an ongoing separate study.
There are limited and somewhat controversial data on CD123 expression in T-ALL. Several prior studies have suggested that CD123 is not or only scarcely expressed in T-ALL, although most of these studies had relatively small sample sizes.13,15,18 Two other studies reported expression of CD123 by T-ALL blasts.30,31 In a study by Lhermitte et al. CD123 expression was detected in 16% of T-ALL; notably, CD123 positivity was reported to be less frequent in children than in adults (9% versus 23%).31 Another study of a large cohort of T-ALL patients by Du et al. found a slightly higher frequency of CD123 expres- sion and noted an age-dependent difference, further sug- gesting a lower frequency of CD123 expression in pedi- atric patients (27% versus 42%).30 A crucial difference precluding direct comparison of the results of those stud- ies with the current study is lack of data on CD123 inten- sity (RFI) in the earlier studies.
ETP-ALL comprises a unique subset of T-ALL previ- ously belonging to the subset of early T-ALL. ETP ALL
Table 2. In vitro cytotoxicity of IMGN632 in B-acute lymphoblastic leukemia/lymphoma cell lines.
Cell line
Genetic variant
IMGN632 Ab-DGN549 (IC50 pM) (IC50 pM)
20 20,000
10 20,000 6 13,000 3 20,000 2 7,000 2 20,000
0.6 3,000
CCRF-SB EBNA-positive
JM1
KOPN-8 t(11;19)(q23;p13)/KMT2A-MLLT1
SEM t(4;11)/KMT2A-AFF1
380 t(8;14)/MYC-IGH and t(14;18)/IGH-BCL2 EBNA-negative TOM-1 t(9;22)(q34.1;q11.2)/BCR-ABL1 fusion gene (e1-a2) SD-1 t(9;22)(q34.1;q11.2)/BCR-ABL1 fusion gene (e1-a2)
IC50: half maximal inhibitory concentration; pM: pico-males.
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